This project is to determine the molecular basis of how sympathetic nerves extend axons along the vasculature with the goal of innervating their target organs. To study this, we use the coronary vasculature model as a favorable model system. We discovered a reciprocal guidance event in the patterning of sympathetic nerves in the developing heart. Our whole-mount imaging revealed that the pattern of large-diameter coronary veins influences the pattern of sympathetic innervation in the heart. Further genetic studies and in vitro organ culture experiments demonstrated that coronary veins serve as an intermediate template that guides distal sympathetic axon projection via local secretion of NGF by coronary vascular smooth muscle cells (Nam et al. 2013). This coordinated process ultimately allows topological targeting of axons to final targets such as coronary arteries and the cardiac conduction system. Our results suggest that target organs possess unique and stereotypical patterns of innervation, mediated by tissue substructures, such as coronary veins in the heart, that are adapted to complex organ structure and physiology. We are currently exploring the regulatory mechanisms by which coronary VSMCs guide patterns of sympathetic innervation and tackle two intriguing unresolved questions in neuronal development: how does guidance factor expression shift from central to peripheral blood vessels (e.g. from subepicaridial veins to myocardial arteries) in parallel with axon extension along these vessels; what determines the choice of either innervation to blood vessels or axon extension along blood vessels en route to their final targets. These studies will provide a molecular mechanistic framework for the developmental program underlying vascular control of neuronal guidance and innervation patterns.